1. Field of the Invention
The present invention relates to a semiconductor device with a multi-level structured insulator formed on a semiconductor substrate and its fabrication method, which enables one to realize a reliable gate insulator of metal-oxide-semiconductor (MOS) type semiconductor devices.
2. Description of the Prior Art
With an MOS-type ultra-large-scale integrated circuit device (ULSI), it is well known that the property of a gate insulator of an MOS field-effect transistor (MOSFET) seriously affects the performance of the ULSI. Thus, the reliability improvement of the gate insulator has been an important problem and therefore, various formation methods of this insulator have been researched and developed.
Recently, in consideration with the fact that nitrided silicon dioxide (i.e., nitrided SiO.sub.2) enables to avoid both the shift of the flatband voltage and the increase of the interfacial state density, which are caused by the hot carrier injection, the nitrided SiO.sub.2 has been tried to be used for this purpose and has been considered as a promising material.
On the other hand, a p.sup.+ -type polysilicon film heavily doped with a p-type impurity such as boron (B) has been tried to be used as the gate electrode of the p-channel MOSFET (PMOSFET). The nitrided SiO.sub.2 is capable of restraint of the doped impurity into the p.sup.+ -type polysilicon film from diffusing into the silicon substrate with which the nitrided SiO.sub.2 is contacted and as a result, it has been considered as a promising material for this purpose, also.
A conventional fabrication method of a semiconductor device of this sort, which has a two-level structured insulator formed on a semiconductor substrate, was disclosed in the Japanese Non-Examined Patent Publication No. 2-18934 published in January 1990. A conventional semiconductor device fabricated using this conventional method is shown in FIG. 1. This conventional method will be explained below, which includes rapid thermal nitridation and rapid thermal annealing processes.
First, a semiconductor substrate 21 is thermally oxidized to thereby produce an oxide film 22 on a main surface of the substrate 21. The film 22 is made of an oxide of the semiconductor constituting the substrate 21.
Next, the substrate 21 with the oxide film 22 is subjected to a rapid thermal process using radiated heat in a nitriding atmosphere such as an NH.sub.3 atmosphere, thereby nitriding the oxide film 22. Nitrogen (N) is introduced into the oxide film 22 during this process. Although the distribution of the introduced nitrogen in the film 22 varies dependent upon the nitridation temperature and nitridation time, almost all the introduced nitrogen exists in the neighborhood of the interface of the substrate 21 with the oxide film 22. Therefore, a nitrided oxide film 23 of the substrate semiconductor is produced near the substrate-oxide interface, as shown in FIG. 1.
Finally, the substrate 21 with the films 22 and 23 is subjected to a thermal annealing process using radiated heat in an inert atmosphere, thereby removing hydrogen (H) doped into the oxide film 22 during the prior nitridation process.
Thus, the conventional semiconductor device is obtained as shown in FIG. 1, which has a two-level structured insulator 27 made of the nitrided oxide film 23 formed on the substrate 21 and the oxide film 22 formed on the film 23. As shown in FIG. 1, the thickness of the nitrided oxide film 23 is much less than that of the oxide film 22.
Another conventional fabrication method of a semiconductor device of this sort, which has a three-level structured insulator formed on a semiconductor substrate, was disclosed in the article: Journal of Applied Physics, Vol. 65, No. 2, pp 629-635, Jan. 15, 1989, entitled "compositional Study of Ultrathin reoxidized nitrided oxides". This conventional method includes a thermal reoxidation process performed in an oxygen atmosphere instead of the hydrogen-removing thermal annealing process in the above conventional method of FIG. 1, which is shown in FIGS. 2A to 2D.
First, a semiconductor substrate 31 shown in FIG. 2A is thermally oxidized to thereby produce an oxide film 32 on a main surface of the substrate 31, as shown in FIG. 2B. The film 32 is made of an oxide of the semiconductor constituting the substrate 31.
Next, the substrate 31 with the film 32 is subjected to a thermal nitridation process in an NH.sub.3 atmosphere, thereby nitriding the oxide film 32. Nitrogen is introduced into the oxide film 32 during this process and exists in the surface area of the film 32 and in the neighborhood of the interface of the substrate 31 with the oxide film 32. Therefore, upper and lower nitrided oxide films 33 and 34 of the substrate semiconductor are produced, as shown in FIG. 3C.
Thus, the three-level structure made of the lower nitrided oxide film 33 formed on the substrate 31, the oxide film 32 formed on the film 33, and the upper nitrided oxide film 34 formed on the film 32 is obtained.
The upper and lower nitrided oxide films 33 and 34 are quite different in microscopic structure from each other. The nitrogen atoms introduced into the lower film 33 are strongly bonded to the neighboring silicon (Si) and oxygen (O) atoms. This means that these nitrogen atoms and consequently, the lower film 33 is very stable.
On the other hand, the nitrogen atoms introduced into the upper film 34 tend to exist as interstitial and/or dangling atoms and therefore, they are unstable. This means that the upper film 34 also is unstable.
Finally, the substrate 31 with the three films 32, 33 and 34 is thermally reoxidized in an O.sub.2 atmosphere. Since the nitrogen (N) atoms introduced into the upper nitrided oxide film 34 are unstable, not only the hydrogen (H) atoms but also the nitrogen atoms introduced into the upper film 34 during the prior nitridation process are removed outward. Thus, the upper nitrided oxide film 34 disappears in this process and as a result, the thickness of the oxide film 32 increases by a value equivalent to the thickness of the film 34.
On the other hand, since the nitrogen atoms introduced into the lower nitrided oxide film 33 are stable, only the hydrogen (H) atoms introduced into the lower film 33 are removed. Thus, the thickness of the lower nitrided oxide film 33 remains approximately unchanged through this process.
Further, during the reoxidation process, a thin oxide film 35 is produced at the interface of the substrate 31 with the lower nitrided oxide film 33 due to reoxidation of substrate 31.
Thus, the conventional semiconductor device having a three-level structured insulator 37 is obtained, as shown in FIG. 2D. This insulator 37 is composed of the oxide film 35 formed on the substrate 31, the reoxidized, nitrided oxide film 33 formed on the film 35, and the oxide film 32 formed on the film 33. As shown in FIG. 2D, the thickness of the nitrided oxide film 33 is much less than that of the upper oxide film 32.
In the above document disclosing the conventional semiconductor device shown in FIG. 2D, it was reported that the nitrogen atoms were distributed only in the neighborhood of the interface of the substrate 31 with the oxide film 35. This was due to the fact that the oxide film 35 was very thin and consequently, it could be said that only the lower nitrided oxide film 33 existed near the interface of the substrate 31.
As described earlier, the nitrided oxide film 23 or 33 serves as a diffusion or penetration barrier of the doped impurity atoms into the substrate 21 or 31. To realize this penetration-barrier function, it is not necessary to use a pure, uniform nitride film produced through a deposition process such as a low-pressure chemical vapor deposition (LPCVD). Any nitrogen-containing oxide film such as the nitrided oxide film 23 described above is capable of the function. It is sufficient for this function that this nitrogen-containing oxide film has a nitrogen concentration of several atomic percents and a thickness of approximately 1 to 2 nm.
This nitrogen-containing oxide film will restrain the doped impurity atoms from diffusing into the underlying semiconductor substrate through the nitrogen-containing film, and will facilitate the control of the threshold voltage of the MOSFET.
With the above conventional semiconductor devices, the nitrided oxide film 23 or 33 exists in the neighborhood of the interface of the substrate 21 or 31, in other words, the much thicker oxide film 22 or 32 than the nitrided oxide film 23 or 33 is located at the top of the insulator 27 or 37. As a result, when a material doped with an impurity (e.g., boron) is contacted with the insulator 27 or 37, a problem that the doped impurity atoms within the material tend to diffuse into the insulator 27 or 37 occurs. These doped impurity atoms will degrade its property and/or electrical performance.
This problem becomes serious when the insulator 27 or 37 is employed as a gate insulator of a MOSFET. Specifically, the degradation of the property and/or electrical performance of the insulator 27 or 37 will badly affect the performance or characteristics of the MOSFET, thereby deteriorating the fabrication yield and reliability of the MOS-type ULSI.